Where did all the giant beavers go?

Credits to NPR

One of the great mysteries about North America is what killed off woolly mammoths and other exotic animals that roamed the land after the last ice age. Ideas have ranged from a comet impact and climate change to human hunters. A study published Friday in Science Magazine provides new clues about this — cleverly deduced from samples of a fungus that grew on the animals' dung.

After the end of the last ice age, North America was covered with large mammals even more diverse than we see in Africa today. Jacqueline Gill, a doctoral student at the University of Wisconsin in Madison, is fascinated by how North America came to have such diverse wildlife, and why those animals are all gone today. She says the continent was home to mammoths, mastodons, beavers the size of black bears, giant ground sloths, camels and horses, and predators like the American lion, the short-faced bear and the dire wolf.

"By about 11,000 years ago, we lose about half of the animals in North America larger than the size of a German shepherd, and that's a pretty big ecological event," Gill says.

There aren't enough bones left to determine in detail when exactly they all became extinct, but Gill and her colleagues hit upon a nifty method to track the abundance of grazing animals. The scientists study a certain species of fungus, spores of which end up in lake sediments that date back some 15,000 years.

"The spores from this fungus are preserved in the lake, and the fungus only grows on animal dung," Gill says.

When there were many large animals, there was lots of dung. When the animals went away, so did the dung and the fungus that lived on those droppings. So, by studying the amount of this fungus in various layers of lake sediment, Gill and her group have pieced together a timetable that helps resolve how and when these animals died out.

One of the long-held ideas had been that a climate shift caused a change in vegetation, and the new plant matter was no good for these grazing animals, so they starved.

Gill and company have disproved that, however, by putting a date on the animals' decline: It occurred more than 14,000 years ago.

"This happens before some of the widespread vegetation change that we see in the areas that we studied, which suggest to us that habitat loss is actually not a cause of the decline, but possibly a consequence," Gill says.

That is, the vegetation might have changed after the browsing animals disappeared and stopped munching on it.

The new dates also help eliminate other ideas that have been argued over the years. John Alroy at the University of California, Santa Barbara says it's clear that the extinctions couldn't have been caused by a cold snap called the Younger Dryas.

"These data show that the extinction happened well before that time, and there's also been a recent paper arguing that the extinction might have been caused by the impact of a comet, and that impact appears to have happened much, much later than the decline, at least [at] this location," Alroy says.

So what does that leave? Well, human beings were moving into North America at that time, so Alroy says this is more evidence that hunters played a key role in driving these large animals to extinction.

"Now, really the question is: Exactly what did humans do to cause this large extinction?" Alroy says.

Gill is interested in that question, too, because, she notes, human beings are now forcing another mass extinction as we reshape habitat around the world.